I thought it might be a problem with the pump, so I cleaned the skimmer and pump it a few days ago. There doesn't seems to be any problem with the pump.
When I say it skims a little I mean the skimmer will bubble up and produces some skimmate. After a while it would lose all the bubbles like if though someone just puts food in the water and no bubbles for 10 hours or so. And the cycle repeats. But what I did notice is skimmer works a bit better after I've done a partial water change. I think I will do another water change in the next couple of days.
Water parameters are: SG
0, PH 8.3, KH
400, Temp 27C. Basically no difference to before the problem.